Brain Derived Neurotrophic Factor (BDNF) is the neurotrophin that appears to permit survival of gustatory ganglion cells during development. However, whether it also serves to direct axonal ingrowth, or cues ingrowing axons to invade particular target tissues is unclear. Furthermore, BDNF continues to be expressed by adult taste buds and its role in adult life is unknown. The proposed experiments utilize novel experimental approaches to investigate the role of BDNF in both developing and mature taste buds. In vitro techniques will be used to test whether axons emerging from the progenitor cells of gustatory ganglia require BDNF for either targeting or as a cue to invade target tissues. For these experiments the epibranchial placodes, which give rise to the gustatory ganglia of the VII and IX cranial nerves, will be cultured with appropriate and inappropriate epithelium taken from early embryos; i.e., before differentiation of gustatory primordia has begun. Cultures will be treated with function blocking BDNF or control antibodies to assess whether BDNF plays a role in either directing axonal outgrowth or in promoting invasion and innervation of target tissues. Other experiments will utilize ELISA in situ techniques to test whether mature taste buds release BDNF either constitutively or under conditions of stimulation. Finally, we will utilize conditional tissue-specific knockout transgenic mice to test whether elimination of BDN-F in adults has any effect on gustatory ganglion cell survival, or on maintenance of taste buds. For these experiments, we will cross two existing transgenic lines, one carrying a floxed BDNF construct, and the other carrying a tamoxifen-receptor-linked Cre, driven by the cytokeratin 14 promoter. Thus administration of tamoxifen to these adult mice will result in excision of the BDNF coding sequence in all epithelial tissues, including taste buds. By using conditional, tissue specific knockouts, we will be able to separate the effects of elimination of BDNF in adulthood from its trophic effects already documented for the embryonic period.